Temperature sensing circuits are commonly used in integrated circuits to protect against operation at excessive temperatures.
Temperature sensing circuits provide an output signal (typically a voltage level) that varies with temperature. This output signal conventionally provides temperature-dependent feedback to a control circuit in an integrated circuit. By monitoring the temperature-dependent feedback, the control circuit is able to respond to increased operating temperatures by, for example, shutting down the integrated circuit or decreasing the operating speed of the integrated circuit.
Temperature sensing circuits conventionally convert temperature to voltage using the thermal voltage (i.e., KT/q), conventionally expressed as V.sub.T, multiplied by some constant n. Expressed mathematically, such temperature sensing circuits may output a voltage V.sub.O =nV.sub.T that varies with temperature according to a known relationship. At present, the accepted standard for temperature-to-voltage converters calls for a change in output voltage V.sub.O of 10 mV/.degree.C. (i.e., dV.sub.O /dT=10 mV/.degree.C.). However, the thermal voltage V.sub.T changes at only about 0.1 mV/.degree. C. (i.e., dV.sub.T /dT=0.1 mV/.degree.C). Thus, the constant n is selected to be approximately one hundred (i.e., 10 mV/.degree.C.div.0.1 mV/.degree.C.) so that the output voltage V.sub.O varies with temperature at the standard 10 mV/.degree.C.
Unfortunately, conventional temperature-to-voltage converters that provide an output voltage V.sub.O based upon the thermal voltage V.sub.T provide relatively high output voltage levels at relatively high expected operating temperatures. Thus, circuits using such temperature-to-voltage converters impose a fairly high lower limit on the usable power supply voltage. One solution to this problem has been to create a fixed offset for the output voltage V.sub.O using a band-gap generator; however, that solution is expensive, for it requires substantial silicon real estate.
What is needed is a simple, low cost, temperature-to-voltage converter that provides a low output voltage V.sub.O so that it can be operated with a low supply voltage.